This invention relates to telecommunications and, more specifically, to a telephone ring signal detector, which discriminates a telephone ring signal from other signals on a telephone line, and detects the telephone ring signal on the line, during a first quarter cycle of a first ring signal.
In the telephone art, many different electrical voltages are present on a telecommunication line, or a telephone line. The line usually consists of two wires coupled from a Telephone Company Central Office (CO) to at least one telephone device, such as a premise telephone, at a subscriber premise. The CO typically provides a nominal 48 volt direct current (DC) battery voltage between the wires. This provides an electrical interface between the telephone device and the CO, allowing the CO to communicate by electrical signals with the telephone device. During an alerting state, the CO notifies the premise of an incoming telephone call, by transmitting an alternating ring signal voltage between the wires. The ring signal voltage activates an audible alerting device, such as a telephone ringer associated with the premise telephone. The CO may also connect a DC, or an alternating current (AC), telephone line test voltage to the wires, testing for a DC line resistance, or an AC line impedance. These line tests partially insure that the telephone line is in electrical compliance with a Federal Communication Commission (FCC) document, 47 CFR, Part 68. In addition, a high frequency transient voltage can also occur on the line, due to a pulse dialing of the premise telephone. Since telephone ring signal detectors must be unresponsive to line test signals, details of typical line test voltages are next discussed.
The line test voltages generated by the CO include an alternating pre-ring test voltage, which is applied to the line just prior to the first ring signal, and a DC test voltage, which is applied to the line during other routine maintenance periods. Bellcore documents TR-TSY-000821 and TR-NWT-000505 indicate that the CO may activate a per-call subscriber line test on the line, just prior to the first ring signal. The purpose of this pre-ring line test is to detect any excessive voltage on the line and to determine the line impedance between the wires and from each wire to ground. Bellcore document GR-506-CORE further indicates that a Loop Current Feed Open (LCFO) Interval occurs for a time interval including 0.3 seconds. During the LCFO time interval, the nominal CO battery voltage is disconnect from the wires. During the LCFO Interval, each wire may be independently or simultaneously grounded, and a CO battery test voltage may be independently, or simultaneous, applied to each wire with respect to ground. Multiple LCFOs can also occur, separated by at least a 0.1 second time interval when the nominal CO battery voltage is momentarily connected to the wires. An ANSI document T1.401-1993 discusses similar Open Switch Intervals (OSI). When any of these document described tests are applied to the line as the pre-ring line test, the alternating pre-ring voltage can result between the wires. The pre-ring line test can produce a substantial peak pre-ring voltage for a pre-ring test time interval being greater than a quarter cycle time period of the ring signal. According to an Electronic Industries Association EIA-470-A document, the CO can also apply a DC test voltage from 0V to 200V between the wires and from each wire to ground. The purpose of the DC line test is to provide a DC line resistance measurement on a routine basis. Since a DC resistance measurement requires a non-varying voltage, large DC voltages can be applied to the line for a DC test time interval being much greater than the quarter cycle time period of the ring signal. During this routine DC line test, the DC line test voltage may also be switched to and from the wires. For this case, an alternating test voltage is produced across the wires, and having an alternating time period much greater than the quarter cycle time period of the ring signal. In addition, this routine DC line test can produce a peak line test voltage that is comparable to a peak ring signal voltage. Thus, in addition to the nominal 48 DC volts and the ring signal voltage on the line, other line signal voltages include the transient voltage due to pulse dialing, the pre-ring AC line test voltage, and a switched or non-switched DC line test voltage. All of these voltages are possible incoming signals to a telephone ring signal detector coupled to the line. In general, it is desirable for such a ring signal detector to detect the ring signal voltage and to reject all other incoming line signal voltages. Thus such a detector must discriminate the ring signal from other line signals on the line.
Recently, enhanced telephone ring signal detectors have been developed, to provide an automated telephone answering function. These enhanced detectors answer incoming calls without activating any audible ringer mechanisms associated with premise telephones connected to the wires. These enhanced detectors are described in U.S. Pat. No. 5,544,241 to Dibner et. al., on Aug. 6, 1996, in U.S. Pat. No. 5,920,624 to Davis on Jul. 6, 1999, and in patent application Ser. No. 09/096,964 by Davis. These enhanced detectors couple an off-hook terminal impedance to the line, in response to detecting the ring signal, prior to a first quarter cycle time period of a first telephone ring signal. The off-hook impedance is substantially equivalent to a terminal impedance exhibited by a telephone, when a telephone""s receiver is removed from a telephone""s cradle and hook switch. The CO detects an off-hook condition, due to the off-hook impedance on the line, and terminates the first telephone ring signal voltage. As a result, the ring signal voltage cannot achieve sufficient magnitude or energy to activate any of the audible ringer mechanisms connected to the wires. These enhanced detectors further require that other line signals are discriminated from a ring signal, during a discriminating time interval not to exceed the quarter cycle time period of a ring signal. In addition, the ring signal must be detected during a first quarter cycle time period of a first telephone ring signal.
The detectors described by the U.S. Patents to Dibner and Davis can detect the ring signal prior to the first quarter cycle time period of the first ring signal. However, it is possible for both of these detectors to erroneously detect the pre-ring and switched DC line test voltages as a valid first ring signal voltage. This results when these test voltages exceed the threshold voltages of these detectors, for at least a time interval equal to the quarter cycle time interval of a ring signal. Thus the patented detectors by Dibner and Davis cannot discriminate between line test voltages and a valid first ring signal voltage, during a discriminating time interval equal to the quarter cycle time interval of a ring signal. The detector, described by the Davis application Ser. No. 09/096,964, provides an improvement over the Dibner detector, by generating an adaptive predetermined threshold voltage, which is greater than the peak pre-ring voltage and less than the peak ring signal voltage. As a result, this detector, described by the Davis application, discriminates the pre-ring line test voltage from a first ring signal voltage. However, when the DC test voltage is switched to the line and exceeds the adaptive threshold voltage for at least the quarter cycle time interval of a ring signal, the detector, described in the Davis application, would also erroneously detect the DC line test voltage as a valid first ring signal voltage. Thus the detector, described by the Davis application, cannot discriminate between the switched DC switched line test voltage and a valid first ring signal voltage in the discriminating time interval equal to the quarter cycle time interval of a ring signal.
Hence there is a need for an improved telephone ring signal detector, which can discriminate other line signals, such as the line transient and line test voltages, from a first telephone ring signal voltage. This signal discrimination must occur, during a discriminating time interval being less than the quarter cycle time interval of a ring signal. There is a further need for the improved ring signal detector to be unresponsive to the line transient and line test voltages and to only be responsive to the first telephone ring signal voltage. Given the above needs, the ring signal detection must occur during the first quarter cycle time period of the first telephone ring signal.
Accordingly, it is an object of the present invention to provide an improved telephone ring signal detector.
It is a further object of the present invention to provide an improved telephone ring signal detector, which discriminates other line signals from a first telephone ring signal on a telecommunication line, where other line signals include an alternating pre-ring line test voltage, an alternating line test voltage due to a switched DC line test voltage, and a high frequency transient voltage, due to pulse dialing of a premise telephone.
It is another object of the present invention to provide an improved telephone ring signal detector, which discriminates other line signals from a first telephone ring signal on a telecommunication line, during a discriminating time interval being less than a quarter cycle time interval of a telephone ring signal.
It is a still further object of the present invention to provide an improved telephone ring signal detector, which is unresponsive to other line signals on a telecommunication line, yet being responsive to a first telephone ring signal on the telecommunication line, during a first quarter cycle time period of a first telephone ring signal.
It is yet another object of the present invention to provide an improved telephone ring signal detector, which is unresponsive to other line signals on a telecommunication line, yet couples an off-hook terminal impedance to a telecommunication line, during a first quarter cycle time period of a first telephone ring signal.
According to one aspect of the present invention, there is provided a telephone ring signal detector coupled to a telecommunication line for detecting a first telephone ring signal on a telecommunication line, and discriminating the first telephone ring signal from other line signals on the telecommunication line. The telephone ring signal detector comprises an event generator coupled to the telecommunication line, for receiving an incoming signal and for generating an initial event signal and an ending event signal. The telephone ring signal detector further comprises a signal characteristic generator, coupled to the telecommunication line and to the event generator, for generating a signal characteristic of the incoming signal after the initial event signal. The telephone ring signal detector further comprises a signal characteristic limit device, for generating a predetermined signal characteristic range limit of the first telephone ring signal, and a signal characteristic comparator, coupled to the signal characteristic generator, to the signal characteristic limit device, and to the event generator, for comparing the signal characteristic with the predetermined signal characteristic range limit. The signal characteristic comparator generates a valid compare signal in response to the signal characteristic being within the predetermined signal characteristic range limit. The signal characteristic comparator also generates a ring detect signal, during the first quarter cycle time period of the first telephone ring signal, in response to the valid compare signal and the ending event signal having occurred.
According to a second aspect of the present invention, the telephone ring signal detector further comprises a line impedance device for providing an off-hook terminal impedance. The line impedance device is coupled to the telecommunication line and to the signal characteristic comparator for coupling the off-hook terminal impedance to the telecommunication line in response to the ring detect signal. As a result, an off-hook condition on the telecommunication line occurs during the first quarter-cycle time period of the first telephone ring signal.